The present invention relates to systems for controlling wheel spin in a motor vehicle such as an automobile, and in particular to such a system which includes a pump-back adaptive braking system and a traction control system which utilize a common pump and wheel cylinder pressure modulating valves.
Adaptive braking systems, also variously denominated anti-lock braking systems and anti-skid braking systems, are now well known. Early braking systems of this type were typically mechanical systems utilizing devices such as a momentum wheel or shifts in a vehicles suspension geometry to sense a change in the coefficient of friction between a vehicles wheel and a surface indicative of an excessive slip condition. Upon sensing such a condition, braking pressure was released or reduced to thereby avert locking of the wheel with an attendant decrease in braking distance and skids. Subsequently, such systems have been continuously developed with electronic wheel speed sensors, electronic circuitry, and high speed brake pressure modulating valves now being used to monitor and control the rotational behavior of the vehicles wheels during braking.
It is now further recognized that a vehicles braking system can also be utilized to absorb torque of the vehicles driven wheels to thereby reduce or avert spinning of the vehicles wheels during acceleration. Such systems are commonly referred to as traction control systems. Since both types of systems in effect control the amount of slip of a vehicles wheels on a road surface, a system incorporating both adaptive braking and tracion control may be and is herein denominated a wheel slip control system.
At the present time adaptive braking systems can be subdivided into two basic categories. Specifically, these include replenishment systems in which a motor driven pump provides the power boost for the braking system and also provides a source of pressurized brake fluid during adaptive braking operation and "pump-back" systems in which a motor driven pump is intermittently operated to replace braking fluid depleted from the vehicles wheel cylinder during adaptive braking operation.
Of these systems, the pump-back systems have the advantages of allowing the use of well known and highly developed vacuum boosters, potentially lower cost, and some inherent failure mode advantages. Heretofore, however, pump-back systems have had the advantage of not being readily adaptable for traction control. This is due to the absence of a source of pressurized braking fluid, typically provided by a high pressure source during adaptive braking, when the vehicle is in a non-braking mode. In view of the above enumerated advantages of pump-back systems, there exists a need for a wheel slip control system which possesses the advantages of a pump-back adaptive braking system and which simultaneously provides for traction control.
Broadly, the present invention is a wheel slip control system which comprises speed sensor means operatively connected to the wheels of a vehicle for generating a signal as a function of the rotation thereof. A control means is connected to the sensors and responsive to the rotational behavior of the wheel for generating control signals in response to wheel slip. Means including a master cylinder and at least one wheel brake cylinder are provided for applying braking force to the vehicles wheels. Brake pressure modulating means are connected in the braking circuit for automatically modulating the application of braking force to the wheels in response to the control signals and means for generating braking and traction controls signals in response to the application of braking and acceleration force to the wheel, respectively. A pump is provided and is operable in a first state to pump braking fluid from a pressurized source typically the wheel cylinder, during brake anti-skid activity, and operable in a second state to draw and pump braking fluid from an unpressurized source, typically the master cylinder reservoir. First valve means are provided and operable in response to a traction control signal for blocking communication from the wheel cylinder to the master cylinder and for providing communication therebetween in the absence of the traction control signal. A second valve means is provided for connecting the pump means to a source of unpressurized braking fluid in response to the traction control signal and blocking communication therebetween the absence of the traction control signal.
The dual operating state pump and associated first and second valve means enable the use of a common pump for both adaptive braking and traction control in a pump-back type adaptive braking system wherein the normal pump-back type of pump must not draw a vacuum to ensure proper operation of the system during adaptive braking.
It is therefore an object of the invention to provide an improved wheel slip control system.
Another object of the invention is to provide such a system wherein common elements of a pump-back adaptive braking system also function to provide traction control.
Still another object of the invention is to provide such a system incorporating a dual operating state pump operable during adaptive braking to pump pressurized fluid to restore braking fluid lost from a wheel cylinder during adaptive braking and operable in a second state to pump braking fluid from a non-pressurized source to provide a source of pressurized fluid for traction control.
Yet another object of the invention is to provide such a system which includes a pump provided with novel porting and a minimum number of additional control valves to enable a pump-back adaptive braking system to also function as a traction control system.